Chemical conversion reagent for magnesium alloy, surface-treating method, and magnesium alloy substrate

ABSTRACT

The objective of the present invention is to impart coating film adhesion, corrosion resistance and rust prevention to magnesium alloys.  
     A chemical conversion reagent for a magnesium alloy which comprises a phosphate ion and a permanganate ion and has pH of 1.5 to 7.

FIELD OF THE INVENTION

[0001] The present invention relates to a chemical conversion reagent for magnesium alloys which imparts high corrosion resistance, rust prevention, and coating film adhesion, a surface-treating method which comprises using said chemical conversion reagent, and a magnesium alloy substrate obtained by the method. The surface-treating method of the invention is suitable for the surface treatment of magnesium alloys which are used in applications requiring corrosion resistance, rust prevention, coating film adhesion, and electrical conductivity, for example electronic components such as portable telephones and personal computer housings, and household electrical appliances such as television receivers. In accordance with the surface-treating method of the invention, the high oxidizing power of a permanganate ion is utilized to encourage formation of a magnesium oxide coat and, at the same time, the reaction of permanganate and magnesium ions with a phosphate ion is utilized to form a coat comprising a phosphorus-manganese compound and a phosphorus-magnesium compound, whereby a chemical conversion film having excellent corrosion resistance, rust prevention, and coating film adhesion can be produced on the magnesium alloy.

BACKGROUND OF THE INVENTION

[0002] The common method of chemical conversion treatment for a magnesium alloy comprises dipping the magnesium alloy in a chemical conversion reagent containing a hexavalent chromium as described in JIS-H8651 and MIL-M-3171, for instance. However, hexavalent chromium exerts harmful effects on the environment and human physiology. Therefore, the working environment for the surface treatment is unfavorable and a sophisticated effluent disposal system is required for preventing discharge of the hexavalent chromium into the environment. The use of hexavalent chromium has the disadvantage that, in addition to said harmful effects on the environment and human physiology, an additional capital expenditure is required for the surface-treating equipment. Furthermore, it is likely that its use will be controlled or prohibited by law in many countries of the world in near future. Therefore, development of a chemical conversion treatment technology for magnesium supplanting chromating treatment, the so-called nonchromate chemical conversion treatment technology, is awaited.

[0003] Regarding such nonchromate chemical conversion treatment methods, some of them are described in Japanese Kokai Publication Hei-07-126858. Thus, a treatment method using a magnesium phosphate treatment as a base and a phosphate treatment method involving additional use of a metal other than chromium, such as zirconium, titanium or zinc, are mentioned as the prior art. However, as pointed out in Japanese Kokai Publication Hei-07-126858, these surface-treating methods are disadvantageous in that they are not practical because of a time-consuming treatment step, involve a long time for the treatment and/or cannot impart sufficient corrosion resistance, rust prevention or coating film adhesion, etc.

[0004] As a technology overcoming the above disadvantages, Japanese Kokai Publication Hei-07-126858 discloses a manganese phosphate treatment. This manganese phosphate treatment method is characterized in that not only a bivalent manganese ion (Mn²⁺) and a phosphate ion but also an aliphatic amine, an aromatic amine, or a heterocyclic amine compound is formulated in a chemical conversion bath. Furthermore, this solution is optionally supplemented with one member selected from a nitrate ion, a sulfate ion and a fluorine-containing compound. It is described that by this treatment method, a coat having excellent corrosion resistance, rust prevention and coating film adhesion can be formed on a magnesium alloy. However, addition of an amine compound is not an eco-friendly practice and, moreover, the composition of this treating solution is so much complicated that bath control is rendered difficult.

[0005] Japanese Kokai Publication Hei-08-35073 discloses a chemical conversion treatment method employing a permanganate ion. It is stated that by adding a promoter such as a mineral acid or a fluoride to an aqueous permanganate solution, a chemical conversion coat having satisfactory corrosion resistance can be formed. However, the coat so formed is not a manganese phosphate coat but a coat composed of manganese oxide and manganese hydroxide, not containing a phosphate bond, and, therefore, is not fully satisfactory in coating film adhesion.

SUMMARY OF THE INVENTION

[0006] The prior art mentioned above has several drawbacks, namely (1) hexavalent chromium exerting harmful effects on human physiology and the environment is contained, (2) a treatment step is time-consuming and complicated, (3) a high temperature is required for the treatment, (4) corrosion resistance, rust prevention and coating film adhesion cannot be imparted as satisfactory as for the chromated coat, and/or (5) bath composition is complicated. The object of the present invention is to overcome these disadvantages.

[0007] The present invention relates to a chemical conversion reagent for a magnesium alloy

[0008] which comprises a phosphate ion and a permanganate ion and

[0009] has pH of 1.5 to 7.

[0010] The concentration of a compound serving as a source of said phosphate ion is preferably 20 to 50 g/L and the concentration of a compound serving as a source of said permanganate ion is preferably 1 to 10 g/L.

[0011] The present invention further relates to a surface-treating method

[0012] which comprises a step of bringing said chemical conversion reagent into contact with a magnesium alloy substrate.

[0013] Said magnesium alloy substrate is preferably subjected to degreasing, pickling, and desmutting treatments in advance and, more preferably, said pickling treatment is carried out with a reagent containing at least one member selected from the group consisting of sulfuric acid, nitric acid, phosphoric acid, and a fluorine-containing compound.

[0014] The present invention is further directed to a magnesium alloy substrate obtainable by said surface-treating method.

[0015] The present invention is now described in further detail.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The chemical conversion reagent for a magnesium alloy according to the invention comprises a phosphate ion and a permanganate ion. The role of said phosphate ion is that of imparting corrosion resistance and improving coating film adhesion through formation of a phosphoric compound. The source of said phosphate ion includes, for example, phosphoric acid salts such as primary sodium phosphate, secondary sodium phosphate, primary ammonium phosphate, secondary ammonium phosphate, primary potassium phosphate, secondary potassium phosphate, etc. and orthophosphoric acid. The concentration of the phosphoric acid compound in the chemical conversion bath may range from 5 g/L to the solubility limit of the compound to be used but is more preferably 20 g/L to 50 g/L. If it is less than 5 g/L, no sufficient coating film adhesion will be obtained. If it exceeds 50 g/L, substantially no further improvement in performance will be obtained but rather will result in a loss.

[0017] The role of said permanganate ion is that of promoting formation of an oxide film on the surface of a magnesium alloy and causing formation of a phosphorus-manganese compound/manganese oxide film which is excellent in corrosion resistance. The permanganate ion is supplied from a permanganate salt compound. Specifically, a permanganate ion can be generated by dissolving potassium permanganate, sodium permanganate, ammonium permanganate, or the like in water. The concentration of the permanganate salt compound in the chemical conversion bath may range from 1 g/L to the solubility limit of the compound to be used but is suitably 1 g/L to 10 g/L. If it is less than 1 g/L, deposition of the manganese compound coat will be insufficient so that no adequate corrosion resistance may be obtained. If the concentration exceeds 10 g/L, no further improvement in performance will be obtained.

[0018] The chemical conversion reagent for a magnesium alloy according to the invention has pH of 1.5 to 7. In accordance with the invention, excellent corrosion resistance and coating film adhesion can be obtained over a broad pH range of 1.5 to 7. The pH can be mainly controlled by adding an alkaline solution, such as an aqueous sodium hydroxide solution, or an acidic solution, such as a solution of orthophosphoric acid, but can also be controlled by modulating the concentration ratio of primary sodium phosphate and secondary sodium phosphate. If the pH is below 1.5, dissolution of the magnesium alloy will be so vigorous as to damage the surface thereby no sufficient corrosion resistance will be obtained. If the pH exceeds 7, neither sufficient corrosion resistance nor sufficient coating film adhesion will be obtained, hence it is not suitable. This is probably because the deposited amount of the coat is drastically decreased and the oxidizing power of the permanganate ion is attenuated.

[0019] The chemical conversion reagent of the present invention can suitably be applied to magnesium alloys. The surface-treating method which comprises a step of bringing said chemical conversion reagent into contact with a magnesium alloy substrate is also another aspect of the present invention.

[0020] The magnesium alloys mentioned above are mainly magnesium alloys prepared by a die-casting technique or a thixomolding technique, and as preferred species, AM50D, AM60D, and AZ91D can be mentioned. As other metals which is used in the preparation of magnesium alloys, there can be mentioned, for example, aluminum, manganese, zinc, silver, and rare earth elements, etc.

[0021] These alloys are sometimes severely contaminated by emulsion oils, known as mold release agents, which are used in casting operations, and depending on casting conditions of alloys, segregation layers may occur on surfaces of alloys. For insuring a normal chemical conversion treatment in such cases, the surface of the magnesium alloy needs to be subjected to a suitable pretreatment. The surface treatment is generally carried out in the sequence of degreasing, washing with water, pickling, washing with water, desmutting, washing with water, chemical conversion treatment, washing with water, and drying.

[0022] The degreasing treatment mentioned above is carried out to remove oil from the surface. This degreasing step alone is sufficient when the degree of contamination is low. The degreasing agent to be used in the above degreasing treatment can be roughly classified into an alkaline degreasing agent and an acidic degreasing agent. The degreasing method for use in the practice of the present invention is not particularly restricted but since magnesium dissolves vigorously in an acidic aqueous solution, selective dissolution of the magnesium in the magnesium alloy may possibly take place to damage the surface. Therefore, it is preferable to use an alkaline degreasing agent.

[0023] The pickling treatment mentioned above is carried out to remove the segregation layer composed of fine crystallites which have segregated on the surface and the mold release agent which has penetrated into the segregation layer. In accordance with the method of the invention, it is preferable to use an aqueous solution of orthophosphoric acid supplemented with a fluorine-containing compound such as hydrosilicofluoric acid or an aqueous solution of an inorganic acid such as sulfuric acid or nitric acid. The concentration range of such an acid is preferably 0.3 to 20 g/L, more preferably about 0.3 to 5 g/L. If it is less than 0.3 g/L, repeated use will diminish a cleaning potency life, thus necessitating frequent supplementation or exchange of the solution. If the concentration exceeds 20 g/L, dissolution of the magnesium alloy will be so vigorous that the alloy surface will be damaged and a considerable amount of smut is produced, hence it is not preferable. The temperature of the pickling bath can be controlled within the range of room temperature to not higher than the boiling point of the aqueous acidic solution but for conserving the working environment and avoiding the surface damage due to excessive etching or the excessive formation of smut, the range of room temperature to about 50° C. is preferred. When an aqueous solution of a carboxyl group-containing organic acid is used, a sufficient cleaning effect may not be obtained owing to formation of a compound coat on the surface of the magnesium alloy, which may possibly cause a problem on coating film adhesion. Moreover, an aqueous solution of either orthophosphoric acid or phosphorous acid alone in a concentration of about 1 g/L or higher may phosphorylate the surface of magnesium with the consequent failure to express a sufficient cleaning effect or insure the necessary coating film adhesion.

[0024] The acid to be used is preferably an acid inert to the magnesium alloy, such as sulfuric acid or nitric acid.

[0025] The desmutting treatment mentioned above is carried out to remove the foulant (smut) on the surface of magnesium alloy, and the washing is generally carried out with an aqueous alkaline solution, such as an aqueous sodium hydroxide solution.

[0026] After each of said degreasing, pickling, and desmutting treatments, washing with water is carried out in the per se known manner. The above drying step can also be carried out in the conventional manner.

[0027] Since the surface-treating method of the present invention imparts excellent corrosion resistance, rust prevention and coating film adhesion to magnesium alloy substrates over a broad pH range of 1.5 to 7, it is expected to find application in a broad range of fields for example electronic components such as portable telephones and personal computer housings, and household electrical appliances such as television receivers. The magnesium alloy substrate obtainable by this manner also constitutes another aspect of the present invention.

[0028] The chemical conversion reagent for a magnesium alloy according to the present invention can impart coating film adhesion and corrosion resistance to magnesium alloys with high reproducibility over a broad pH range of pH 1.5 to 7 so that it facilitates bath control and handling. This is a major industrial merit.

[0029] The surface-treating method of the present invention can impart excellent corrosion resistance and coating film adhesion to magnesium alloys, even to magnesium alloys prepared by a casting technique and hence contaminated with a mold release agent. The method, therefore, can be used in a broad range of applications such as personal computer and portable telephone housings and other shaped articles, parts having intricate profiles, automotive parts, and so forth.

EXAMPLES

[0030] The following Examples illustrate the present invention in further detail. However, these are by no means limitative to the scope of the invention.

Example 1

[0031] In a sequence of degreasing, washing with water, pickling, washing with water, desmutting, washing with water, manganese phosphate treatment, washing with water, and drying, each carried out under the following conditions, a chemical conversion coat was formed on a magnesium alloy. Magnesium alloy: AZ91D test pieces (size: 100 mm×50 mm×3 mm)

[0032] Degreasing: A 1 weight % aqueous solution of “Surf Fine 100” (an alkaline degreasing agent: product of Nippon Paint Co.); bath temperature 50° C., treating time 2 min. Pickling: Orthophosphoric acid 0.4 g/L, hydrosilicofluoric acid 0.03 g/L, tap water as balance; bath temperature 50° C., treating time 2 min

[0033] Desmutting: Sodium hydroxide 20 g/L, sodium gluconate 3.1 g/L; bath temperature 60° C., treating time 5 min. Chemical conversion treatment: KMnO₄ concentration 5.5 g/L, primary phosphate concentration 45 g/L, orthophosphoric acid concentration 1.8 g/L, tap water as balance; pH 2.8, bath temperature 50° C., treating time 2 min Drying condition: Drying in an oven at 100° C. for 10 min

[0034] The uncoated corrosion resistance, coated corrosion resistance, coating film adhesion, and coat appearance were evaluated and the surface electrical resistivity was measured. The evaluations of parameters other than coated corrosion resistance were made on the above magnesium alloy test piece obtained and the evaluation of coated corrosion resistance was made on the test piece prepared by further applying a powder coating on each of the above magnesium alloy test pieces obtained. The results are presented in Table 1.

[0035] (1) Evaluation of Corrosion Resistance

[0036] The uncoated corrosion resistance (the corrosion resistance of a magnesium alloy after chemical conversion treatment) was evaluated by the salt spray test (SST). SST is a test in which the test piece is sprayed with a 5 weight % aqueous sodium chloride solution for a predetermined time in a tester controlled at 35° C. and the incidence of corrosion is then evaluated. The evaluation of the corrosion resistance was made by visual assessment of uncorroded area after 48 hours of SST exposure.

[0037] (2) Coated Corrosion Resistance

[0038] Coated corrosion resistance was evaluated by SST. The coating film was cross-cut and set in an SST tester for a predetermined time (96 h). A cellophane tape was pressed against the surface of the cross-cut portion and, then, peeled off as defined and the maximum peel width from the cut portion was measured. The smaller the peel width is, the better the coated corrosion resistance is. As the coating, a gray epoxy powder coating (Magdyne PD-E, product of Nippon Paint Co.) was used. The curing conditions of coating films were 160° C. and 20 minutes. The dry film thickness of the coating film was 40 μm.

[0039] (3) Coating Film Adhesion

[0040] The coating film adhesion was evaluated according to a warm-water immersion test. The warm-water immersion test is a method in which a sample is immersed in warm water at 50° C. for a predetermined time (24 h, 96 h) and the coating film adhesion is then evaluated. The coating film adhesion in this occasion was evaluated by the cross-hatch test method which comprises cross-hatching the coating film into 100 squares, 1 mm×1 mm each, pressing a cellophane tape against the cross-hatched surface, peeling the tape off as defined, and counting the remaining coating film squares.

[0041] (4) Measurement of Surface Electrical Resistivity

[0042] The surface electrical resistivity was measured by the two-terminal method. The measurement was made in 9 points per sample and, among the values found, 7 values to the exclusion of the maximum and minimum values were averaged to find surface resistivity value. For measurements, a surface electrical resistivity tester (EP-T360, manufactured by Keyence) was used.

[0043] (5) Evaluation of Coat Appearance

[0044] The appearance was evaluated visually. TABLE 1 Coated Uncoated corrosion corrosion resistance Coating film adhesion resistance Blister Number of remaining Surface Coat Uncorroded width coating film squares resistivity appearance area (%) (mm) 24 h 96 h (Ω) (color) Ex. 1 70% 0 100 100 0.4 Brown Ex. 2 80% 0 100 100 10    Reddish brown Ex. 3 80% 0 100 100 0.4 Pale brown Compar. Ex. 1 80% 0 0 — — Very pale brown Compar. Ex. 2 40% 0 0 — — White Ex. 4 80% 0 100 100 0.1 Pale yellow Ex. 5 80% 0 100 100 0.1 Pale brown Compar. Ex. 3 20% 0 100 100 0.4 Pale gray Compar. Ex. 4 30% 7 10  5 0.1 Pale yellow Compar. Ex. 5 50% 20 0  0  0.08 White

[0045] Except that 75 weight % orthophosphoric acid was added to the same chemical conversion bath as used in Example 1 while monitoring with a pH meter to bring the pH to 1.8, the procedure of Example 1 was otherwise repeated to prepare test pieces and evaluation and measurement were carried out.

Example 3

[0046] Except that 20 weight % aqueous sodium hydroxide solution was added to the same chemical conversion bath as used in Example 1 to bring the pH to 6.9, the procedure of Example 1 was otherwise repeated to prepare test pieces and evaluation and measurement were carried out.

Comparative Example 1

[0047] Except that 20 weight % aqueous sodium hydroxide solution was added to the same chemical conversion bath as used in Example 1 to bring the pH to 9, the procedure of Example 1 was otherwise repeated to prepare test pieces and evaluation and measurement were carried out.

Comparative Example 2

[0048] Except that 20 weight % aqueous sodium hydroxide solution was added to the same chemical conversion bath as used in Example 1 to bring the pH to 12, the procedure of Example 1 was otherwise repeated to prepare test pieces and evaluation and measurement were carried out.

Example 4

[0049] Except that, in the pickling step of Example 3, 1 g/L aqueous sulfuric acid solution was used as the pickling bath, the procedure of Example 3 was otherwise repeated to prepare test pieces and evaluation and measurement were carried out.

Example 5

[0050] Except that, in the pickling step of Example 3, 1 g/L aqueous nitric acid solution was used as the pickling bath, the procedure of Example 3 was otherwise repeated to prepare test pieces and evaluation and measurement were carried out.

Comparative Example 3

[0051] Except that, in the chemical conversion treatment step of Example 3, 20 weight % aqueous sodium hydroxide solution was added to 45 g/L aqueous primary sodium phosphate solution while monitoring pH value with a pH meter to bring the pH of the chemical conversion bath to 6.8 and the resulting bath was used, the procedure of Example 3 was otherwise repeated to prepare test pieces and evaluation and measurement were carried out. Here, a chemical conversion reagent not containing a permanganate ion was used and this was intended to verify the effect of a permanganate ion.

Comparative Example 4

[0052] Except that, in the chemical conversion treatment step of Example 3, a reagent comprising 5.5 g/L of the permanganate ion was used in the absence of a phosphate ion, the procedure of Example 3 was otherwise repeated to prepare test pieces and evaluation and measurement were carried out. This was intended to verify the effect of a phosphate ion.

Comparative Example 5

[0053] Except that the chemical conversion treatment and subsequent step of washing with water were not performed among the steps described in Example 1, the procedure of Example 1 was otherwise repeated to prepare test pieces and evaluation and measurement were carried out. 

1. A chemical conversion reagent for a magnesium alloy which comprises a phosphate ion and a permanganate ion and has pH of 1.5 to
 7. 2. The chemical conversion reagent for a magnesium alloy according to claim 1 wherein the concentration of a compound serving as a source of the phosphate ion is 20 to 50 g/L and the concentration of a compound serving as a source of the permanganate ion is 1 to 10 g/L.
 3. A surface-treating method which comprises a step of bringing the chemical conversion reagent according to claim 1 or 2 into contact with a magnesium alloy substrate.
 4. The surface-treating method according to claim 3 wherein the magnesium alloy substrate is subjected to degreasing, pickling and desmutting treatments in advance.
 5. The surface-treating method according to claim 4 wherein the pickling treatment is carried out with a reagent containing at least one member selected from the group consisting of sulfuric acid, nitric acid, phosphoric acid and a fluorine-containing compound.
 6. A magnesium alloy substrate obtainable by the surface-treating method according to any of claims 3 to
 5. 